The present invention relates to a prism system and a liquid crystal projection device, and more particularly to the prism system which is operated fur totally reflecting incident rays of light or for reflecting a ray of light having a predetermined wavelength.
FIGS. 7a and 7b show a conventional prism system where the light going through a prism is reflected to be emerged in a different direction from the incident light.
The system comprises a triangular prism I made of solid glass and having an incident surface 1a, a reflecting surface 1b and an emerging surface 1c. On the reflecting surface 1b, a dichroic mirror 2 is formed by deposition. The dichroic mirror 2 is a multi-layer film comprising a high refractive index layer and a low refractive index layer each of which has a predetermined thickness end alternatively layered in order. The dichroic mirror has spectral characteristics and polarizing characteristics.
A ray of light enters the prism 1 approximately a right angle to the incident surface
The light entered at the incident surface 1a goes through the prism 1 and strikes the reflecting surface 1b. the dichroic mirror 2 reflects light of predetermined wavelength component included in entered light and passes light beams of the other components. For example, one of red (R), green (G) and blue (B) components is reflected from the mirror and two others transmit through the mirror.
The light reflected from the dichroic mirror 2 goes through the prism 1 and strikes the incident surface 1a. The light is again totally reflected from the surface 1a and emerges from the emerging surface 1c.
In such a system, the spectral characteristics the light, namely the characteristics for reflecting and transmitting a desired wavelength component of the incident light is controlled through the dichroic mirror by varying the thickness and the number of layers of the mirror.
Furthermore, it is necessary to control the phase characteristics of the light. Since the transmittance of the S polarized light of the light passing through he multi-layer of the optical thin film is different from that of the P polarized light, a phase difference occurs between the incident light and the reflecting light, making characteristics different in polarized condition.
For example, in a liquid crystal projection device, luminous intensity of light is modulated by a liquid crystal panel, and the light id applied to a projection lens through a prism system. In the device, since light beams emitted from a light source and entered in the prism system are not completely parallel, a part of the rays modulated by the panel is not applied to the projection lens through the prism system. Therefore, the luminance of the light projected on a screen is insufficient.
Accordingly, the dichroic mirror of the prism system is further arranged to have a phase control characteristic so as to completely apply the light passing through the liquid crystal panel to the lens.
Thus, the dichroic mirror of the prism system controls the phase characteristics as well as the spectral characteristics. However, it is necessary to increase the number of layers or provide an additional control layer.
As a result, problems arise on difficulties in controlling the dichroic mirror and manufacturing the thickness of the layer.